1. Field of the Invention
The present invention relates to a stepping motor downsized and capable of smooth operation, and to a stepping motor suitable for, for example, a two-phase electro-motor for a meter installed in a vehicle.
2. Description of the Related Art
In an analog-type meter installed in a vehicle for presenting analog indication, a needle is generally driven by a cross-coil motor. However, in recent years, a stepping motor has been applied in place of the cross-coil motor in order to improve directional accuracy and realize downsizing.
Specifically, as a stepping motor for an in-vehicle meter, those described in Translated National Publication of Patent Application No. Hei 11-501800, U.S. Pat. No. 6,043,574, and so on are known. These have a structure in which, as shown in FIG. 13 and FIG. 14, a rotor 112 is magnetized in a manner that its magnetic poles change along a circumferential direction, and excitation coils 116A and 116B are respectively provided on two supporting portions 114A and 114B which are stators respectively having a W-shape provided in a magnetic yoke 114. Further, in the structure, two magnetic circuits having a W-shape are composed by magnetic polar pieces 118 parts of which are communized.
On the one hand, as technique simply related to the stepping motor, disclosed in Japanese Utility model Publication No. Hei 7-27826 is that a pair of magnetic yokes 124A and 124B having a W-shape respectively mounted with excited coils 126A and 126B are separately arranged around the rotor 122, as shown in FIG. 15. On the other hand, also disclosed in Japanese Patent Publication No. Hei 4-74693, Japanese Patent Laid-open No. Hei 8-251902, and so on is technique that magnetic yokes having a U-shape are separately arranged around an outer circumference of a rotor.
In other words, according to these documents, as a general conventional stepping motor, a structure including a rotor composed of a permanent magnet in which a plurality of magnetic poles are sequentially magnetized along a circumferential direction and a pair of magnetic yokes having a U- or W-shape arranged around the rotor has been known.
Meanwhile, although reduction in current has been required for the stepping motor in recent years, space for excitation coils is restricted by the size of bobbins around which excitation coils are wound and magnetic yokes on which the bobbins are mounted (for example, the size L in FIG. 14 and FIG. 15). As a result, in order to realize the reduction in current, it becomes necessary to increase the number of turns for winding wires, decrease the wire size, and so on, but the following problems arise.
Specifically, even when the magnetic yokes having the U- or W-shape and the fixed size are used, the number of the turns of the excitation coils can be increased by making the wire size smaller, but, the smaller the wire size becomes, the greater care must be taken to avoid a break in winding the wires, which deteriorates an assembly property as well as greatly affects reliability due to a possibility of the break even after the winding.
Furthermore, there are disadvantages that, if the number of turns is increased while keeping the wire size as it is, the excitation coils become unable to be inserted into the magnetic yokes having the U- or W-shape, which results in the necessity for upsizing the magnetic yokes and also for upsizing the stepping motor, and the like.
On the other hand, although the movement of the analog indication in which the needle moves smoothly is generally required for meters, the needle does not move smoothly when the conventional stepping motor is applied to the meter because detent torque is large and only discontinuous rotational movement can be obtained in the conventional stepping motor.
Considering the above facts, it is a first object of the present invention to provide a stepping motor capable of freely changing the number of turns of an excitation coil so as to enhance assembly operations and reliability without increasing a magnetic yoke size, and it is a second object of the present invention to provide a stepping motor capable of moving smoothly.
According to one aspect of the present invention, provided is a stepping motor comprising: a rotor having a plurality of magnetic poles along its circumferential direction; one stator base arranged near the rotor; a pair of magnetic yokes fixed on the stator base and arranged to face the magnetic poles of the rotor; a pair of excitation coils mounted on the pair of magnetic yokes respectively, for exciting the pair of magnetic yokes respectively in response to drive pulses having phase difference from each other; and a pair of stator magnetic polar portions integrally formed with the stator base, for forming magnetic paths together with the magnetic yokes respectively.
With the stepping motor, the following operation is performed.
Specifically, according to the aspect, the stepping motor is structured in which the stator base is arranged near the rotor having the plurality of magnetic poles along its circumferential direction, and the pair of magnetic yokes arranged to face the magnetic poles of the rotor are fixed on the stator base.
Further, according to the aspect, as the drive pulses having the phase difference from each other are applied to the pair of excitation coils which are respectively mounted on the pair of magnetic yokes, the pair of excitation coils excite the pair of magnetic yokes respectively. Then, the pair of stator magnetic polar portions, which are integrally formed with the stator base and separated from each of the magnetic yokes, form the magnetic paths together with the magnetic yokes and are magnetized when the pair of magnetic yokes are excited.
In other words, according to the aspect, since the stator magnetic polar portions which form the magnetic paths together with the magnetic yokes are formed separately from the magnetic yokes, the magnetic yokes can be formed in, for example, an I-, T-, or J-shape, which does not restrict the size of the excitation coils.
As a result, different from the magnetic yokes having the U- or W-shape, the restriction on the shape of the magnetic yokes in arranging the excitation coils is reduced, and the number of turns of the excitation coils can be easily increased. Therefore, flexibility of resistance values of the excitation coils applied to the stepping motor according to this aspect greatly increases, which also improves flexibility in designing a shape of the stepping motor.
As described above, according to this aspect, it becomes possible to freely change the size of the excitation coils without increasing the size of the magnetic yokes, and assembly operations and reliability of the stepping motor can be enhanced while decreasing the size of the stepping motor.
Moreover, according to this aspect, since the stator base and the pair of stator magnetic polar portions are integrally formed, the stepping motor can be assembled while positioning the magnetic yokes, which are fixed to the stator base, and the stator magnetic polar portions easily with high precision. Thus, the stator magnetic polar portions can be manufactured more easily by presswork and the like, which can reduce the manufacturing cost as well as forms an accurate magnetic circuit.
Meanwhile, as an example of the aspect, the stepping motor can be considered to have a structure in which, when an angle between the pair of magnetic yokes around a center of the rotor is xcex1, angles between the magnetic yokes and the stator magnetic polar portions around the center of the rotor are xcex21 and xcex22, and an angle between the pair of stator magnetic polar portions around the center of the rotor is xcex3,
and if the number of magnetic poles of the rotor is N, K is an integer, a is an odd number, and b is 0 or 1, each of the angle xcex1 and angles xcex21 and xcex22 is less than 180xc2x0 and obtained from formulas of
xcex1=(K+0.5)xc3x97360/N 
xcex21,xcex22=(axc2x1b/4)xc3x97360/N. 
However, in a case where a value of C is obtained from a formula of C=xcex3xc3x97N/360 is an integer, detent torque increases. Therefore, the angles are controlled so that C is not an integer.
In other words, it can be considered that the pair of magnetic yokes are fixed to the stator base in positions under the above-described conditions and the stator magnetic polar portions are arranged in positions under the above-described conditions with respect to the magnetic yokes. As a result, owing to the positions of the pair of magnetic yokes and the pair of stator magnetic polar portions, magnetic forces given from the magnetic poles of the rotor when the excitation coils are not energized cancel out each other so that the detent torque is reduced.
As described above, in this example, similarly to the above description, it becomes possible to freely change the size of the excitation coils without increasing the size of the magnetic yokes, and assembly operations and reliability of the stepping motor can be enhanced while decreasing the size of the stepping motor, as well as the detent torque is reduced so that the rotor moves smoothly.
As a result, if the stepping motor according to this example is applied to the meter, the needle moved by the stepping motor operates smoothly.
According to another aspect of the present invention, provided is a stepping motor having a structure in which permeability of material composing the pair of magnetic yokes is higher than permeability of material composing the stator base, in addition to the structure similar to that in the aforesaid aspect of the present invention.
With the stepping motor, the following operation is performed.
Specifically, according to the aspect, although the operation is the same as that in the aforesaid aspect of the present invention, a large magnetic force can be generated with a small current owing to the above-described relation of permeability, which allows the stepping motor to function optimally. Further, it becomes possible to use high-permeability material whose cost is generally high only in the minimum required part and the manufacturing cost for the stepping motor is further reduced.
According to still another aspect of the present invention, provided is a stepping motor having a structure in which a case body for forming an outer frame of the stepping motor is included and a stator fixing hook is formed in the case body so that the stator base is fixed to the case body by the stator fixing hook, in addition to the structure similar to that in the aforesaid aspect of the present invention.
With the stepping motor, the following operation is performed.
Specifically, according to the aspect, although the operation is the same as that in the aforesaid aspect of the present invention, in assembling the stepping motor, the stator base engages with the stator fixing hook so as to be fixed to the case body only by pushing the stator base into the case body. Therefore, the manufacturing steps are simplified and the manufacturing cost for the stepping motor is further reduced.